subterranean

/sˌʌbtɪɹˈe‍ɪni‍ən/

/səbtɝˈeɪniən/

subterranean

lying beyond what is openly revealed or avowed (especially being kept in the background or deliberately concealed)

being or operating under the surface of the earth

Examples

1. This war is subterranean.

2. In Rome, subterranean catacombs housed both cremation urns and intact remains.

3. This subterranean fungus is one of the most expensive ingredients in the world.

4. These subterranean microbes don't need oceans or sunshine.